Flexible printed circuit board

ABSTRACT

A flexible printed circuit board with reduced ion migration from signal-carrying elements which are coated against corrosion includes an insulating layer, a wiring area, a copper electroplating layer, a nickel electroplating layer, a cover film, and a gold chemical-plating layer. The wiring area is formed on the insulating layer. The copper electroplating layer formed on the wiring area has a first portion and a second portion. The nickel electroplating layer is formed on at least the first portion and exposes sidewalls of the first portion. The cover film is formed on the second portion and fills in gaps of the copper electroplating layer. The gold chemical-plating layer is formed on top surface of the nickel electroplating layer and the sidewalls of the first portion.

FIELD

The subject matter herein generally relates to circuit boards, andparticularly, to a flexible printed circuit board (FPCB) and a methodfor making the FPCB.

BACKGROUND

FPCBs are widely used in various kinds of electronic devices. The FPCBusually comprises a copper wiring layer. A surface of the copper wiringlayer is usually treated by chemical plating of nickel and gold to forma nickel chemical-plating layer and a gold chemical-plating layer,thereby increase corrosion resistance of the FPCB.

However, when the copper wiring layer has a minimal line space, the linespace may further decrease due to existence of the nickelchemical-plating layer, which may increase the risk of ion migrationfrom the copper wiring layer.

BRIEF DESCRIPTION OF THE DRAWINGS

Implementations of the present technology will now be described, by wayof example only, with reference to the attached figures.

FIG. 1 is a flowchart of an exemplary embodiment of a method for makingan FPCB.

FIG. 2 is a diagram of a carrier used in the method of FIG. 1.

FIG. 3 is a diagram showing a first photoresist layer being formed onthe carrier of FIG. 2.

FIG. 4 is a diagram of a copper electroplating layer formed byelectroplating of copper in the first photoresist layer of FIG. 3.

FIG. 5 is a diagram showing a second photoresist layer being formed onthe first photoresist layer and the copper electroplating layer of FIG.4.

FIG. 6 is a diagram of a nickel electroplating layer formed byelectroplating nickel in the second photoresist layer of FIG. 5.

FIG. 7 is a diagram showing the first and the second photoresist layersof FIG. 6 removed.

FIG. 8 is a diagram showing a non-wiring area of the carrier of FIG. 7etched and removed.

FIG. 9 is a diagram showing a cover film being formed on the copperelectroplating layer of FIG. 8.

FIG. 10 is a diagram of an exemplary embodiment of an FPCB formed bychemical plating gold on the nickel electroplating layer of FIG. 9.

FIG. 11 is a diagram of another exemplary embodiment of an FPCB formedby chemical plating nickel and gold on the nickel electroplating layerof FIG. 9.

FIG. 12 is a flowchart of another exemplary embodiment of the method formaking an FPCB.

FIG. 13 is a diagram showing a second photoresist layer formed on thefirst photoresist layer and the copper electroplating layer in themethod of FIG. 12.

FIG. 14 is a diagram of a nickel electroplating layer formed byelectroplating nickel in the second photoresist layer of FIG. 13.

FIG. 15 is a diagram showing the first and the second photoresist layersof FIG. 14 removed, non-wiring area of the carrier etched, a cover filmcovering the copper electroplating layer, and a gold chemical-platinglayer formed on the nickel electroplating layer, to form yet anotherexemplary embodiment of an FPCB.

DETAILED DESCRIPTION

It will be appreciated that for simplicity and clarity of illustration,where appropriate, reference numerals have been repeated among thedifferent figures to indicate corresponding or analogous elements. Inaddition, numerous specific details are set forth in order to provide athorough understanding of the embodiments described herein. However, itwill be understood by those of ordinary skill in the art that theembodiments described herein can be practiced without these specificdetails. In other instances, methods, procedures, and components havenot been described in detail so as not to obscure the related relevantfeature being described. Also, the description is not to be consideredas limiting the scope of the embodiments described herein. The drawingsare not necessarily to scale and the proportions of certain parts may beexaggerated to better illustrate details and features of the presentdisclosure.

The disclosure is illustrated by way of example and not by way oflimitation in the figures of the accompanying drawings, in which likereferences indicate similar elements. It should be noted that referencesto “an” or “one” embodiment in this disclosure are not necessarily tothe same embodiment, and such references mean at least one.

The term “comprising,” when utilized, means “including, but notnecessarily limited to”; it specifically indicates open-ended inclusionor membership in the so-described combination, group, series, and thelike.

Referring to FIGS. 1-10, a method for making an FPCB 100 (shown in FIG.10) is presented in accordance with a first exemplary embodiment. Themethod for making the FPCB 100 is provided by way of example, as thereare a variety of ways to carry out the method. The exemplary method canbegin at step 101.

At step 101, referring to FIG. 2, a carrier 10 is provided, whichcomprises an insulating layer 11 and a copper foil 12 formed on at leastone surface of the insulating layer 11 (FIG. 1 shows only one copperfoil 12). The copper foil 12 comprises a wiring area 121 and anon-wiring area 122 besides the wiring area 121.

In at least one exemplary embodiment, the insulating layer 11 is made ofa polymer selected from a group consisting of polyimide,polytetrafluoroethylene, poly thiamine, poly (methyl methacrylate),polycarbonate, polyethylene terephthalate, andpolyimide-polyethyleneterephthalate, and any combination thereof. In atleast one exemplary embodiment, the insulating layer 11 has a thicknessof about 10 μm.

At step 102, referring to FIG. 3, a first photoresist layer 20 is formedon the copper foil 12. The first photoresist layer 20 has a first hollowpattern 21 to expose the wiring area 121 of the copper foil 12.

In at least one exemplary embodiment, the first photoresist layer 20 istreated by an exposure and development process to form the first hollowpattern 21. The first photoresist layer 20 can be a dry film.

At step 103, referring to FIG. 4, the first hollow pattern 21 iselectroplated with copper, thereby forming a copper electroplating layer30 on the wiring area 121. The copper electroplating layer 30 comprisesa first portion 31 that awaits nickel electroplating and a secondportion 32 besides the first portion 31.

In at least one exemplary embodiment, the copper electroplating layer 30has a thickness of about 25 μm.

At step 104, referring to FIG. 5, a second photoresist layer 40 isformed on and covered on the first photoresist layer 20 and the copperelectroplating layer 30. The second photoresist layer 40 has a secondhollow pattern 41 to expose at least the first portion 31 of the copperelectroplating layer 30.

In at least one exemplary embodiment, the second photoresist layer 40covers only the first portion 31.

In at least one exemplary embodiment, the second photoresist layer 40 istreated by an exposure and development process to form the second hollowpattern 41. The second photoresist layer 40 can also be a dry film.

At step 105, referring to FIG. 6, the second hollow pattern 41 iselectroplated with nickel, thereby forming a nickel electroplating layer50 a at least on the first portion 31.

In at least one exemplary embodiment, the nickel electroplating layer 50a has a thickness of about 2 μm to about 6 μm.

At step 106, referring to FIG. 7, the first photoresist layer 20 and thesecond photoresist layer 40 are removed.

At step 107, referring to FIG. 8, the non-wiring area 122 of the copperfoil 12 is etched and removed so that the wiring area 121 forms adesired circuit wiring.

At step 108, referring to FIG. 9, a cover film 60 is covered on thesecond portion 32 of the copper electroplating layer 30, thereby causingthe cover film 60 to fill in gaps of the copper electroplating layer 30and the cover film 60 to be spaced from sidewalls of the first portion31.

In at least one exemplary embodiment, the cover film 60 is a solder masklayer.

At step 109, referring to FIG. 10, a top surface of the nickelelectroplating layer 50 a facing away from the first portion 31 and thesidewalls of the first portion 31 are chemical plated with gold to forma gold chemical-plating layer 70, thereby forming the FPCB 100.

FIG. 11 illustrates in another exemplary embodiment, before forming thegold chemical-plating layer 70, the top surface of the nickelelectroplating layer 50 a facing away from the first portion 31 and thesidewalls of the first portion 31 are chemical plated with nickel toform a nickel chemical-plating layer 50 b. Then, the goldchemical-plating layer 70 is formed on a top surface of the nickelchemical-plating layer 50 b facing away from the first portion 31 andsidewalls of the nickel chemical-plating layer 50 b. In this exemplaryembodiment, the sum of thicknesses of the nickel electroplating layer 50a and the top surface of the nickel chemical-plating layer 50 b is about2 μm to about 6 μm. The sidewall of the nickel chemical-plating layer 50b has a thickness of about 0.2 μm to about 2 μm.

Referring to FIG. 12, a second exemplary method for making an FPCB 100′is further provided. Differences from the above first exemplaryembodiment are:

At step 104′, referring to FIG. 13, a second photoresist layer 40 isapplied on the first photoresist layer 20 and the copper electroplatinglayer 30. The second photoresist layer exposes both the first portion 31and the second portion 32 of the copper electroplating layer 30.

At step 105′, referring to FIG. 14, the second hollow pattern 41 iselectroplated with nickel, thereby forming a nickel electroplating layer50 a both on the first portion 31 and the second portion 32.

Thus, at step 108′, referring to FIG. 15, the cover film 60 covers thesecond portion 32 of the copper electroplating layer 30 and the nickelelectroplating layer 50 a formed on the second portion 32.

Depending on the exemplary embodiment, certain of the steps of methodsdescribed may be removed, others may be added, and the sequence of stepsmay be altered. It is also to be understood that the description and theclaims drawn to a method may include some indication in reference tocertain steps. However, the indication used is only to be viewed foridentification purposes and not as a suggestion as to an order for thesteps.

FIGS. 10 and 11 illustrate that in the first exemplary embodiment, anFPCB 100 comprises an insulating layer 11 and a wiring area 121 formedon at least one surface of the insulating layer 11. A copperelectroplating layer 30 is formed on and corresponds to the wiring area121. The copper electroplating layer 30 comprises a first portion 31 anda second portion 32 besides the first portion 31. A nickelelectroplating layer 50 a is formed on a top surface of at least thefirst portion 31 facing away from the wiring area 121. The nickelelectroplating layer 50 a exposes sidewalls of the first portion 31. Acover film 60 is formed on the second portion 32 and fills in gaps ofthe copper electroplating layer 30.

In at least one exemplary embodiment, the nickel electroplating layer 50a is only formed on the top of the first portion 31.

Referring to FIG. 10, in at least one exemplary embodiment, a goldchemical-plating layer 70 is formed on a top surface of the nickelelectroplating layer 50 a facing away from the first portion 31 and thesidewalls of the first portion 31.

Referring to FIG. 11, in at least one exemplary embodiment, a nickelchemical-plating layer 50 b is formed on a top surface of the nickelelectroplating layer 50 a facing away from the first portion 31 and thesidewalls of the first portion 31. The gold chemical-plating layer 70 isformed on a top surface of the nickel chemical-plating layer 50 b facingaway from the first portion 31 and sidewalls of the nickelchemical-plating layer 50 b.

FIG. 15 illustrates a second exemplary embodiment of an FPCB 100′.Differences from the first exemplary embodiment are that the nickelelectroplating layer 50 a is formed on a top surface of both the firstportion 31 and the second portion 32 facing away from the wiring area121. The nickel electroplating layer 50 a exposes sidewalls of the firstportion 31 and the second portion 32.

With the above configuration, the nickel electroplating layer 50 a isnot formed on the sidewalls of the first portion 31, thereby preventingthe line space of the copper electroplating layer 30 from being reducedby the nickel electroplating layer 50 a. Thus, a risk of ion migrationfrom the copper electroplating layer 30 is reduced. Even when the nickelchemical-plating layer 50 b is formed on the top surface of the nickelelectroplating layer 50 a facing away from the first portion 31 and thesidewalls of the first portion 31, a very small thickness of the nickelchemical-plating layer 50 b can prevent ion migration from the copperelectroplating layer 30, thereby preventing the line space of the copperelectroplating layer 30 from being greatly reduced.

Moreover, the nickel electroplating layer 50 a can function as acatalytic agent in the process of chemical-electroplating of nickel.Thus, during the process of chemical-electroplating of nickel and gold,the copper electroplating layer 30 does not need to be activated bypalladium substitution. That is, palladium residue within the nickelelectroplating layer 50 a is avoided, which further reduces the risk ofion migration.

Even though information and advantages of the present embodiments havebeen set forth in the foregoing description, together with details ofthe structures and functions of the present embodiments, the disclosureis illustrative only. Changes may be made in detail, especially inmatters of shape, size, and arrangement of parts within the principlesof the present embodiments to the full extent indicated by the plainmeaning of the terms in which the appended claims are expressed.

What is claimed is:
 1. A flexible printed circuit board comprising: aninsulating layer; a wiring area formed on at least one surface of theinsulating layer; a copper electroplating layer formed on andcorresponding to the wiring area, the copper electroplating layercomprising a first portion and a second portion besides the firstportion; a nickel electroplating layer formed on a top surface of atleast the first portion facing away from the wiring area, the nickelelectroplating layer exposing sidewalls of the first portion; a coverfilm formed on the second portion and filling in gaps of the copperelectroplating layer; and a gold chemical-plating layer formed on a topsurface of the nickel electroplating layer facing away from the firstportion and the sidewalls of the first portion.
 2. The flexible printedcircuit board of claim 1, wherein the nickel electroplating layer isonly formed on the top surface of the first portion.
 3. The flexibleprinted circuit board of claim 2, wherein the nickel electroplatinglayer has a thickness of about 2 μm to about 6 μm.
 4. The flexibleprinted circuit board of claim 1 further comprising a nickelchemical-plating layer, wherein the nickel chemical-plating layer isformed on the top surface of the nickel electroplating layer facing awayfrom the first portion and the sidewalls of the first portion, and thegold chemical-plating layer is formed on top of the nickelchemical-plating layer facing away from the first portion and sidewallsof the nickel chemical-plating layer.
 5. The flexible printed circuitboard of claim 4, wherein a sum of thicknesses of the nickelelectroplating layer and the top surface of the nickel chemical-platinglayer is about 2 μm to about 6 μm, and the sidewall of the nickelchemical-plating layer has a thickness of about 0.2 μm to about 2 μm. 6.The flexible printed circuit board of claim 1, wherein the nickelelectroplating layer is formed on a top surface of both the firstportion and the second portion facing away from the wiring area, and thenickel electroplating layer exposes sidewalls of the first portion andthe second portion.
 7. The flexible printed circuit board of claim 1,wherein the insulating layer is made of a polymer.
 8. The flexibleprinted circuit board of claim 7, wherein the polymer is selected from agroup consisting of polyimide, polytetrafluoroethylene, poly thiamine,poly (methyl methacrylate), polycarbonate, polyethylene terephthalate,and polyimide-polyethyleneterephthalate, and any combination thereof. 9.The flexible printed circuit board of claim 1, wherein the cover film isa solder mask layer.
 10. The flexible printed circuit board of claim 1,wherein the copper electroplating layer has a thickness of about 25 μm.